with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve shows several small peaks and then
a large peak at ~T+130 sec with a total duration of at least 160 sec.
The peak count rate was ~24,000 counts/sec (15-350 keV), at ~138 sec after the trigger.
Due to a Sun observing constraint, Swift cannot slew to the BAT
position until 10:53 UT on 2016 September 29. There will thus be no XRT
or UVOT data for this trigger before this time.
Burst Advocate for this burst is M. H. Siegel (siegel AT swift.psu.edu).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

GCN Circular #19831
**J. McEnery, J. Racusin (NASA/GSFC) and Francesco Longo
(INFN/Trieste) report on
behalf of the Fermi-LAT team:
At 20:36:44.33 on August 21, 2016, Fermi-LAT triggered
on high-energy emission from GRB 160821A, also detected by GBM (trigger
493504474/
GRB160821857), and Swift-BAT (Siegel et al., GCN 19830).
The onboard location is
RA, Dec = 172.5, 43.0 (J2000)
with an error radius of 0.5 deg (90% containment, systematic error
only). This was 17 deg
from the LAT boresight at the time of the trigger.
Onboard LAT GRB detections are relatively rare, and imply that this GRB
is exceptionally
bright at high energy gamma-rays. We strongly encourage follow-up
observations.
The Fermi-LAT point of contact for this burst is Francesco Longo
(francesco.longo@trieste.infn.it ).
The Fermi-LAT is a pair conversion telescope designed to cover the
energy band from 20
MeV to greater than 300 GeV. It is the product of an international
collaboration between
NASA and DOE in the U.S. and many scientific institutions across
France, Italy, Japan and
Sweden.

GCN Circular #19832
Gendre B. (UVI - Etelman Obs.), Klotz A., Turpin D., Atteia J.L.
(CNRS-OMP-IRAP), Boer, M., Laugier, R. (CNRS-ARTEMIS) report:
We imaged the field of GRB 160821A detected by SWIFT
(trigger 709351) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.
The observations started 29.31min after the GRB trigger.
The elevation of the field decreased from
10 degrees above horizon and weather conditions
were fair.
We detect a new fading source in the error box given by SWIFT (Siegel et
al., GCNC 19830) at the following position (+/- 3 arcsec):

RA(J2000.0) = 11h 25m 01.6s
DEC(J2000.0) +42d 20' 05"

OT was R~15.4 at 29.31min after GRB. Due to the brightness of the event,
we encourage any follow-up.
Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.
N.B. Galactic coordinates are lon=166.5065 lat=+66.7370
and the galactic extinction in R band is 0.1 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.
This message may be cited.

GCN Circular #19835
M. Stanbro (UAH) and C. Meegan (UAH) report on behalf of the Fermi GBM Team:
"At 20:34:30.04 UT on 21 August 2016, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 160821A (trigger 493504474 / 160821857), which
was also detected by the Swift/BAT (Siegel et al. 2016, GCN 19830) and
by the LAT (McEnery et al. 2016, GCN 19831).
The GBM on-ground location is consistent with the Swift position.
The GBM light curve shows/consists of several episodes and
a bright burst with a duration (T90) of about 43 s (50-300 keV).
The time-averaged spectrum from T0-4.1 s to T0+194.6 s is
best fit by a Band function with Epeak = 968 +/- 16 keV,
alpha = -1.08 +/- 0.00, and beta = -2.38 +/- 0.03
The event fluence (10-1000 keV) in this time interval is
(550.3 +/- 1.39)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+135.7 s in the 10-1000 keV band
is 123.1 +/- 0.7 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

GCN Circular #19836
M. Arimoto (Waseda U./Tokyo Tech), M. Axelsson (KTH Stockholm), F.
Dirirsa (U. Johannesburg) and F. Longo (INFN/Trieste)
report on behalf of the Fermi-LAT team:
We report the on-ground localization and analysis of GRB 160821A,
which triggered
an onboard LAT detection (McEnery et al., GCN 19831). All times are
relative to the initial GBM
trigger (Stanbro et al., GCN 19835).
The best LAT on-ground location is found to be:
RA, Dec = 171.3, 42.3 deg (J2000)
with an error radius of 0.08 deg (90 % containment, statistical error
only). This is fully
compatible with the position of the prompt emission detected by
Swift/BAT (Siegel et al., GCN 19830).
This was 17 deg from the LAT boresight at the time of the GBM trigger
and triggered
an autonomous repoint of the spacecraft.
More than 50 photons above 100 MeV and 4 photons above 1 GeV were
detected within 245s,
before the spacecraft entered the SAA. The GRB did not come back into
the Fermi-LAT FoV until T0 + 1380s.
The LAT emission was coincident with the bright pulse observed by GBM
at ~T0+135 s.
The highest-energy photon is a 4.7 GeV event which is observed ~212
seconds after the GBM trigger.
The Fermi-LAT point of contact for this burst is Francesco Longo
(francesco.longo@trieste.infn.it).
The Fermi-LAT is a pair conversion telescope designed to cover the
energy band from
20 MeV to greater than 300 GeV. It is the product of an international
collaboration
between NASA and DOE in the U.S. and many scientific institutions across France,
Italy, Japan and Sweden.

with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 79%.
The mask-weighted light curve shows a complex structure with many
overlapping pulses that starts at ~T-3 s and extends beyond T+117s,
when the the burst went out of the BAT FOV due to a pre-planned slew.
The burst flux was rising during the slew, so it is possible that the BAT
detector missed the peak emission. The main peak that is visible in
the BAT light curve occurs at ~T+114 s.
The time-averaged spectrum from T-2.61 to T+115.17 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.52 +- 0.06. The fluence in the 15-150 keV band is 7.2 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+113.51 sec in the 15-150 keV band
is 2.8 +- 0.6 ph/cm2/sec. All the quoted errors are at the 90% confidence
level.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/709351/BA/

GCN Circular #19842
A. Kozlova, S. Golenetskii, R. Aptekar, D. Frederiks, P. Oleynik,
M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline
on behalf of the Konus-Wind team, report:
The long-duration, very bright GRB 160821A
(Swift detection: Siegel et al., GCN 19830; Fermi LAT detection:
McEnery et al., GCN 19831, Arimoto et al., GCN 19836;
Fermi GBM observation: Stanbro et al., GCN 19835)
triggered Konus-Wind at T0=74179.687 s UT (20:36:19.687).
The burst light curve shows a weak emission starting at ~T0-113 s
followed by the bright main episode at ~T0. A total duration of
the burst is ~320 s. The emission is seen up to ~10 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB160821_T74179/
As observed by Konus-Wind, the burst
had a fluence of 1.17(-0.03,+0.03)x10^-3 erg/cm2,
and a 64-ms peak flux, measured from T0+22.320 s,
of 1.06(-0.13,+0.13)x10^-4 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst
(measured from T0 to T0+99.328 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.00(-0.02,+0.02),
the high energy photon index beta = -1.99(-0.04,+0.04),
the peak energy Ep = 710(-44,+47) keV
(chi2 = 212/88 dof)
The spectrum near the maximum count rate
(measured from T0+22.272 to T0+22.528 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.84(-0.10,+0.12),
the high energy photon index beta = -1.89(-0.29,+0.17),
the peak energy Ep = 1074(-326,+431) keV
(chi2 = 54/57 dof)
All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.

GCN Circular #19865
P. S. Marrocchesi (U of Siena), A. Yoshida, T. Sakamoto, Y. Kawakubo,
M. Moriyama, Y. Yamada (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA),
I. Takahashi (IPMU), Y. Asaoka, S. Ozawa, S. Torii (Waseda U),
Y. Shimizu, T. Tamura (Kanagawa U), W. Ishizaki (ICRR),
M. L. Cherry (LSU), S. Ricciarini (U of Florence),
and the CALET collaboration:
The long-duration GRB 160821A (Siegel et al., GCN Circ. 19830;
McEnery et al., GCN Circ. 19831; Stanbro et al., GCN Circ. 19835;
INTEGRAL-SPI/ACS trigger #7537) triggered the CALET Gamma-ray Burst Monitor
(CGBM) at 20:36:21.91 on 21 August 2016. The burst signal was seen by
all CGBM instruments. Note that the CGBM trigger time is 111 sec after
the trigger time of Swift/BAT. This is because the CGBM triggered at the
bright main peak of the burst (Markwardt et al. GCN Circ. 19840).
The light curve of the SGM shows a bright peak with several overlapping
pulses. The emission starts from T sec, peaks at T+24 sec and ends at
T+80 sec. There is a hint of the emission in the SGM light curve around
the trigger time of Swift/BAT. The T90 duration measured by the SGM data
is 35.1 +- 0.8 sec (40-1000 keV).
The light curve is available at
http://cgbm.calet.jp/cgbm_trigger/flight/1155846835/
The CALET data used in this analysis are provided by the Waseda
CALET Operation Center located at the Waseda University.

GCN Circular #19867
V. Bhalerao (IUCAA), V. Kumar (IUCAA), D. Bhattacharya (IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL) report on behalf of the Astrosat CZTI collaboration:
The bright long GRB160821A (Barthelmy et al., GCN Circ. 19830) was detected using the CsI veto detectors on Astrosat CZTI (Bhalerao et al., arXiv:1608.03408). The primary CZTI detectors were disabled as the satellite was passing through the South Atlantic Anomaly at that time.
The source was clearly detected in the 40-200 keV energy range. The light curve shows a single peak, with the strongest peak at 20:36:45.00 UT, 133 seconds after BAT Trigger at 20:34:28 UT. The time delay arises from detection of the main peak of the burst, and the timing is consistent with the Fermi LAT (McEnery et al., GCN 19831) and CALET (Marrocchesi et al., GCN 19865). The peak count rate in the CsI detectors was 11200 counts/sec above the background (four quadrants summed together), with a total of 149300 counts. The local mean background count rate was 1400 counts/sec. Using cumulative rates, we measure a T90 of 42 secs.
CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb . CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.

GCN Circular #20007
Soulier J.-F. (Observatoire de Maisoncelles, France) repports:
I imaged the field of GRB 160821A detected by SWIFT
(trigger 709351) with the Newtonian 30cm F/3.8 telescope
located at the Maisoncelles observatory, France (IAU C10).
I detected the optical transcient described by Gendre et al.
(GCNC 19832) at the following position (+/- 0.5 arcsec):

Taking account for the magnitude provided by Gendre et al.
(GCNC 19832) I derived an optical decay alpha=0.78 for the
first day.
Magnitudes were estimated with the nearby UCAC-4 stars
and are not corrected for galactic dust extinction.